An older type of drill body, which is particularly for machining of aluminium, is disclosed in International Patent Application Publication No. WO 01/91960. However, in this case the tip part of the drill head is extraordinary acute, on one hand, so far that the positive nose angle thereof is limited to approx. 90°, and on the other hand so far that the base width thereof amounts to only approx. ⅓ of the diameter of the drill body. Furthermore, the tip part is fairly thin by being delimited between two face ground surfaces, which are situated comparatively near each other. This results in the mouths of possible cooling liquid ducts having to be located beside the tip part, and therefore the cooling liquid has difficulty to cool and lubricate at least the front portion of the tip part. In other words, the loose-edge formation on the drill body may become troublesome.
A further developed drill body of the type initially mentioned is further known by International Patent Application Publication No. WO 2007/015095 A1. In this case, the tip part has a considerably wider base (50-90% of the diameter), as well as a greater nose angle (preferably 130°) than the tip part of the drill body according to International Patent Application Publication No. WO 01/91960. In such a way, the mouths of the possible cooling liquid ducts can be placed comparatively near the center axis, viz. closer to the same than the periphery. For this reason, the lubricating and cooling liquid can be brought to flush over the two part edges of the tip part in a more efficient way while counteracting loose-edge formation. However, a disadvantage of this further developed drill body is that the individual peripheral part edge, which is negatively angled, has a very small radial extension, as well as is formed on a material portion, the tangential extension of which is limited by the width of the primary clearance surface, because the same extends only up to the borderline of the primary clearance surface against the secondary clearance surface, but not along the last-mentioned one. This means that the material portion in which the peripheral part edge is included will be diminutive and fairly weak. Since the diameter of the drill body is small (e.g., within the range of 5-10 mm) and the primary clearance surface has a width of some tenths of a mm, the material portion in question may easily be damaged or entirely destroyed not only in connection with hole making but also in connection with the handling of the drill body in different situations.
The present invention aims at obviating the shortcomings of the drill body known by International Patent Application Publication No. WO 2007/015095 A1 and at providing an improved drill body. Therefore, an object of the invention to provide a drill body, the peripheral and negatively angled part edges of which are strong to give the drill body an optimum service life, and which is particularly suitable for the machining of aluminium, in particular soft aluminium having low content of silicon, without giving rise to troublesome loose-edge formation.
Another object of the invention is to provide a drill body that can operate with moderate cutting forces and that centers in a reliable way in connection with the entering of a workpiece to be machined.
Yet another object of the invention is to provide a drill body that minimizes the risk of chip jamming and burr formation, as well as reduces not only the axial cutting forces but also the radial ones. In other words, the drill body should be possible to be fed into the workpiece to be machined in a smooth way without the tool being pressed out radially to too great an extent.